This invention relates to a shock absorbing device for a hydraulic cylinder capable of imparting to a piston the function of absorbing the force of shocks in a plurality of stages at the terminating portion of a stroke of the piston of the hydraulic cylinder.
In the majority of hydraulic cylinders, it is usual practice to move the piston rod assembly at high speed to increase operation efficiency. The piston rod assembly moving at high speed has high kinetic energy, so that it is necessary to provide means for absorbing high energy of inertia to bring same to a halt at the end of its stroke. If the piston rod assembly were allowed to impinge on the end wall of the cylinder when it is brought to a halt, a high force of impact would be exerted on the end wall to thereby cause considerable damage thereto. Thus, a shock absorbing device for absorbing the energy of inertia possessed by the piston rod assembly has been provided to absorb the force of shocks at the end of the stroke of the piston.
One type of shock absorbing device is disclosed in Japanese patent application Laid-Open No. 35478/72 (corresponding to U.S. application Ser. No. 128,822). This shock absorbing device comprises an axially extending cylindrical shock absorbing port formed in the end wall of the cylinder housing communicating at one end with the cylinder chamber and at the other end with a suction and exhaust passageway, and a cylindrical shock absorbing member mounted on the piston and adapted to be inserted in the shock absorbing port at the end of the stroke of the piston to reduce the area of the channel in the shock absorbing port. The device functions such that high resistance is offered to a stream of working fluid discharged, in the terminating stages of the stroke of the piston, from the cylinder chamber through the shock absorbing port by the piston as the shock absorbing member enters the shock absorbing port, to thereby restrict the flow rate of the discharged fluid to impart a shock absorbing function to the piston. A disadvantage of this proposed device resides in the fact that the effectiveness of the shock absorbing function may vary depending on the relation between the length of the cylindrical portion of the cylindrical shock absorbing member and the length of the shock absorbing port, and to increase the shock absorbing function would require an increase in these lengths. This however, would increase the overall length of the cylinder. Conversely, in the case of a cylinder of restricted cylinder length, it would be necessary to forego the benefit of shock absorbing function. A further disadvantage resides in the fact that the shock absorbing device has a shock absorbing characteristic such that the instant the shock absorbing member enters the shock absorbing port, deceleration of very high order would take place in the piston and no great deceleration would occur thereafter. Stated differently, the device would only perform a shock absorbing function or energy absorbing function in a single stage. Thus, a very high force of impact would be exerted on the hydraulic cylinder the instant the shock absorbing member enters the shock absorbing port, and a high impact force would be applied to the end wall of the cylinder housing when the piston impinges thereon when it is brought to a halt. Another disadvantage resides in the fact that the provision of the axially extending shock absorbing port and the suction and discharge passageway communicating with the end portion of the shock absorbing member in the end wall of the cylinder housing would increase the axial length of the end wall of the cylinder housing. Moreover, it is only in the annular throttling passageway defined between the inner peripheral surface of the shock absorbing port and the outer peripheral surface of the shock absorbing member that the shock absorbing function is performed, so that the clearance between the inner and outer peripheral surfaces constituting the throttling passageway would exert great influences on the shock absorbing performance. Thus, it would become necessary to increase the precision with which working and assembling are performed, which would be troublesome.